Thank God for Science:

Airplane trails, which are also called contrails, are simply artificial clouds that originate from the exhaust of an airplanes engines. Just like other clouds, airplane trails are composed of water, H2O, either in the form of water droplets (condensation) or ice crystals. The water originates from the combustion (i.e., burning) of airplane fuel (kerosene or gasoline), which also produces carbon dioxide (CO2). The same thing happens when you drive your car on a cold day. The cloud like mist or water droplets coming out of a vehicles exhaust pipe on a cold day is just the water produced from burning fuel in the engine. When it is cold, this water condenses from vapor (a gas) into a liquid in the form of clouds or droplets. That is why you can see car exhaust on cold days. On warm days, the water remains in the vapor (gaseous) state and is invisible, so you cannot see exhaust on hotter days.

Airplane contrails remain in the sky for the same reason that clouds stay in the sky. Clouds consist of water in the form of tiny liquid droplets and can either evaporate into an invisible gas (water vapor), which would cause the cloud to disappear, or can remain as droplets, which causes the cloud to stay there. Whether the water droplets that compose a cloud evaporate (and the cloud disappears) and how long the cloud remains before evaporating depends upon the relative humidity of the air. Relative humidity is a measure of how much water vapor is in the air compared to how much water vapor the air can hold. It is calculated by dividing the amount of water vapor in the air by the maximum amount of water vapor the air can hold.

YOU SHOULD HAVE LEARNED THIS SHIT IN MIDDLE SCHOOL